The present invention relates to springs and more particularly, to springs formed from elongated strip materials without bending and having a biasing portion with undulations, indentations or the like, to enable deflections in a longitudinal axial direction and along the thickness of the spring.
Springs are used in numerous applications for storing and displacing energy. Several types of springs exist; those most commonly known are coil, leaf, disc, spiral springs, and the like, which are used in numerous tensioning and compression applications. Generally, these type of springs work very well for their intended purpose.
When an application is best served by a spring, having flat planar characteristics, which is able to deflect in a longitudinal axial direction, the above disclosed springs will not qualify for this type of application unless additional money is expended to adapt them to the application. Generally, when elongated, flat, planar springs are used, the applications are like those where leaf springs are commonly used. Leaf springs deflect transverse to the longitudinal axis of the elongated strip member, and thus, are unable to perform deflection in a longitudinal axial direction. While coil and spiral springs are capable of deflecting along their longitudinal axes, these springs require additional space and are incapable of being positioned in a narrow, flat, planar opening. Thus, there exists a need in the field for a spring exhibiting flat planar characteristics, while having good resilient and strength properties, and is able to deflect in a longitudinal axial direction. Serpentine shaped springs exist which would operate within these space limitations, but these springs are formed by bending round or square wire into the serpentine shape. The spring disclosed herein is manufactured from a wide strip, by removing materials to achieve the serpentine shape.
Accordingly, it is an object of the present invention to overcome the disadvantages of the above art. The present invention provides the art with a flat, planar, elongated strip spring which enables deflection i a longitudinal axial direction. The present invention enables relatively strong springs to be formed from thin, metallic strip material. The present invention enables deformation of the spring to occur along its width dimension, as well as its cross-section dimension, which, in turn, enables the spring constant of the spring to be easily chosen for a desired application. The present invention provides the art with a relatively strong spring which utilizes a relatively small amount of material with respect to the strength of the spring. Also, the spring exhibits a non-constant or variable force constant which is useful in many tension and compression applications.
The new and improved spring of the present invention is generally formed from a metallic strip material. The spring includes an elongated, flat, metallic strip having a primary biasing portion formed by removal of material, or other means, with undulations, indentations or the like integrally formed from the strip to additionally gain or enhance the primary and/or secondary biasing response. The primary biasing portion enables axial deflection of the spring along the longitudinal axis and along the cross-section direction of the elongated strip. The spring may also include a pair of end members for enabling securement of the spring. The secondary biasing portion is derived from deflection of the strip material by bending across the thickness direction in the manner of a leaf type spring.
Generally, the primary biasing portion of the spring is formed in a serpentine configuration. The serpentine configuration is an integral part of the elongated strip member and may extend the entire length of the strip or only a portion of the strip. The serpentine configuration is formed in the strip by removing the outer portions of the elongated strip between the curved and leg portions of the serpentine configuration or the spring is molded or cast in the shape, so that each element of the serpentine has an "U" shape configuration.
Also disclosed is a method of manufacturing the spring of the present invention. The method includes providing an elongated, metallic strip material and forming an integral biasing portion with undulations, indentations or the like in the strip for enabling axial longitudinal primary deflection and cross-sectional secondary deflection of the spring. The method also includes forming end members on the elongated strip by bending, stretching, as well as punching, etc., and when desired to produce special force constant results, by similar treatment to the "U" sections. The forming of the biasing portion further includes stamping an integral serpentine configuration section into the elongated strip in the embodiments that are shown.
Also disclosed are design and performance parameters of a serpentine strip spring which are somewhat different in tension and compression than springs heretofore used.
Serpentine strip springs are unique in that they may exhibit two or even three spring constants (nominally force/extension), instead of the single spring constant exhibited by springs which have been manufactured by bending the material from which the spring is formed. To exhibit multiple force constant, two modes of flexure are encouraged by the shape of the serpentine strip spring. One flexure is the primary across the broad dimension of the shape, and the other is the secondary across the metal thickness. The serpentine spring, by definition, has a width which is much greater than its thickness (by a factor of 3 or more of the "U" elements) to form the tight "S" shape of the "U" elements of the serpentine strip spring.
The stretching of a serpentine strip spring takes place as follows: First, there is bending or flexing across the primary broad width dimension. Since the material being bent is quite wide, relative to the physical size of the spring, a high force constant is generated. The secondary surfaces of the spring are essentially planar at the time of the initial deformation, other secondary deformations then occur as bending in the thickness direction which alter the original force constant. For explanation purposes, the spring is comprised of reversing "U" elements. As the legs of the "U" are pulled apart, the legs and base of the "U" flex and the angle between the legs increases from a reference zero degree unstretched position. It should be noted that the original width of the strip controls the overall length of the legs of successive "U" elements, and thus, controls the extension of the spring for any given angle between the legs of the "U" elements greater than the unstretched zero degree position.
Second, as stretching continues, the stiffness contribution of the metal thickness begins to be overcome by the compressive resistance of the outside periphery of the base of the "U" elements. At this point, "puckering" or a departure from a planar surface begins to occur at the outside of the "U" elements and twisting begins to occur in the legs of the "U" elements; this is the secondary response. The interior radius of the "U" base is under considerable tension. This is a result of the "lever" forces exerted by the legs which, in turn, form considerable compression in the exterior radius of the "U" base. It is important to note that at this time, no part of the serpentine strip spring is deformed beyond the "Hookes law" region. Also at this time, the spring has a spring constant which is a combination of the original primary mode, planar width deformation, and a secondary mode, bending and twisting of the "U" elements.
The point at which the outer radius of the "U" base begins to depart from the planar surface and starts to "pucker" is strongly influenced by the ratio of width of the strip to thickness of the strip. Altering this ratio and also including certain formed portions to control the stiffness across the material width or apparent thickness direction controls the onset and character of the "pucker" and twisting of the "U" elements. Thus, a spring can easily be designed with a quickly rising resistance to initial extension, having a large force constant in the first mode, giving way to a much more compliant lower force constant in the second mode. The design is executed in a manner wherein extension is halted prior to failure, as is the case with any spring.
Failure of the serpentine strip spring takes place in two stages. First, bending failure occurs in the base of the "U"'s; where folding ("puckering" and twisting under the forces imposed by the legs) occurs causing the strip to be pulled straight. This failure continues until the base of the "U"'s become folded. Folding continues until the strip has been pulled out straight. At this point, the tensile force on the inside radius of the "U" base begins to be the final limiting factor. Final failure occurs when the interior radius of the "U" base finally tears.
To develop design characteristics of serpentine strip springs, a finite element analysis should be done on the force, and resistance to the force, that is occurring during the transition from mode one bending primary, to mode one bending plus mode two secondary puckering and twisting. For given metal properties, to determine a desired mode one spring constant and a desired mode one "pre-load", which is before the onset of mode one plus mode two spring constant, a choice of the width to thickness ratio and the width of the serpentine strip spring elements must be determined. Also, the shape of the "U" legs and base will influence the onset of mode one plus mode two bending since the legs and base resistance to twist also impacts on the onset of "puckering" as well as the extension of the spring do to a given force. It must be appreciated that when the "U" legs and base are deformed but still planar, that the angle between the legs is greater than the unstressed angle. This stressed angle is only a few degrees. The length of the legs (dictated by the width of the original strip) is thus very important to the amount of extension for a given angle. When the "puckering" of the "U" base occurs, there is suddenly a larger increase in this angle per unit of added force, and the spring now extends (or compresses) much more. This is, of course, the onset of a new lower force constant.
Deriving samples to confirm the theory and findings is fortunately fairly straight forward. It is not necessary to make a stamping die for each spring configuration. Instead, the strip thickness and width are chosen, and manufactured by utilizing "chemical milling" or laser cutting processes. The spring may be quickly and inexpensively produced in the desired flat shapes compared to stamping die costs for small quantities.
Serpentine strip springs are very simple in concept, but are relatively complicated in practice because of the interrelationships of all the factors which influence the final result. Failure to fully understand these relationships most likely explains serpentine strip spring's historical non-use. Until one recognizes what can be done with these types of springs, the tendency is to declare the spring a waste of time.
The use of a spring with more than one spring constant, that is essentially a flat strip of material, which stretches like any heavy spring at first, and then while continuing to apply the initial force, stretches further with a much less increase in load, evokes thoughts of where can a device be used. Some immediate possibilities are metal "bungee" straps; special clamps; preloading devices for applications having movement beyond the capabilities of single constant springs within the space available; shock absorbing drive "chains" that are not chains at all but a strip of metal, using a universal joint type drive strip with encoders for angular position to obtain a dynamic torque measurement and a whole family of unique compression springs, which are made by rolling the strip along its long axis into a tubular shape.
Consideration of application areas leads one to the conclusion that in all sorts of products there are mechanical, hydraulic, and pneumatic assemblies that may be replaced by the new spring at a considerable savings of space and cost.
From the following description and claims taken in conjunction with the accompanying drawings, other objects and advantages of the present invention will become apparent to one skilled in the art.